Split cards in M13 will be chase Uncommons/Rares that provide decks with all-upside utility. We would make Split Cards for all 10 color pairs because player's like complete cycles, and Split Cards allow for an exception to the Core mantra "focus on ally pairs" without requiring any other enemy color support. This is the best choice because they are visually appealing, very collectable cycles (good for sales,) almost always top-down designs, an easy way to include multicolor concepts in Core, fun to play with and fun to design.
Search your library for an artifact card, reveal it, and put it into your hand. Then shuffle your library.
Search your library for a sorcery card, reveal it, and put it into your hand. Then shuffle your library.
As I said you don't need to do anything else in the set to support these - they are strictly an added bonus to attract players; however I can see a new dual land cycle that might like them (or other multicolor cards.)
Wilderness Colony (Rare)
As Wilderness Colony enters the battlefield, you may reveal a green card and a white card from your hand. If you don't Wilderness Colony enters the battlefield tapped,
T: Add G or W to your mana pool.